1. Field of the Invention
The present invention relates to the oxide sintered body, a manufacturing method therefor, a manufacturing method for transparent conductive film using the same, and the resultant transparent conductive film, and more specifically the present invention relates to the oxide sintered body substantially containing zinc, tin and oxygen, useful as a target, which can be sputtered under charging of high DC power, and a manufacturing method for an oxide transparent conductive film formable in high-speed, and the oxide transparent conductive film excellent in chemical resistance.
2. Description of the Prior Art
A transparent conductive film is widely used not only as electrodes of solar batteries or liquid crystalline display elements and other various light-sensitive elements, but also as heat reflecting films for automotive windows or construction applications, antistatic films, or antifogging transparent heat generators such as for freezer show cases, because of having high conductivity and high transmittance in the visible light range.
As such transparent conductive films, for example, a tin oxide (SnO2)-based thin film, a zinc oxide (ZnO)-based thin film, and an indium oxide (In2O3)-based thin film are known.
As a tin oxide-based transparent conductive film, one containing antimony as a dopant (ATO) or one containing fluorine as a dopant (FTO) is well utilized. In addition, as a zinc oxide-based transparent conductive film, one containing aluminum as a dopant (AZO) or one containing gallium as a dopant (GZO) is well utilized. The most industrially utilized transparent conductive films are indium oxide-based ones; among them, indium oxide -which contains tin as a dopant is called ITO (indium tin oxide), which is widely used, because a transparent conductive film particularly having low resistance can easily be obtained.
Transparent conductive films having low resistance are suitably used as solar batteries, liquid crystals, surface elements for organic electro luminescence and inorganic electro luminescence, or touch panels and the like.
As a method for manufacturing these transparent conductive films, a sputtering method or an ion plating method is well utilized. In particular, a sputtering method is an effective method for film formation of a material with low vapor pressure, or in the case where precise control of film thickness is required, and widely utilized industrially because of extremely simple and convenient operation.
A sputtering method uses a target as a raw material of a thin film. The target is a solid containing metal elements configuring a thin film to be formed, and a sintered body of such as a metal, a metal oxide, a metal nitride, and a metal carbide, and in some cases, single crystals are used. A sputtering method, in general, uses an apparatus having a vacuum chamber which is capable of arranging a substrate and the target therein. Gas pressure inside the vacuum chamber is controlled to equal to or lower than about 10 Pa, by once making to high vacuum after arrangement of the substrate and the target, followed by introduction of rare gas such as argon or the like. Argon plasma is generated by glow discharge between the substrate as an anode and the target as a cathode, subsequently argon cations in the plasma are made to collide with the target as the cathode, and target component particles flicked thereby are deposited on the substrate to form a film.
A sputtering method is classified by a generation method for argon plasma; one using high frequency plasma is called a high frequency sputtering method, and one using DC plasma is called a DC sputtering method.
In general, a DC sputtering method is widely used industrially, because high-speed film formation is provided, a power unit is cheaper and film formation operation is simpler and more convenient, as compared with a high frequency sputtering method. In addition, a DC sputtering method requires use of a conductive target, while a high frequency sputtering method is capable of forming a film using also an insulating target.
Film formation speed in sputtering is in close relation with a chemical bond of a target substance. Sputtering is a phenomenon where argon cations having kinetic energy are collided with the target surface, and substances at the target surface are flicked by receiving the energy, therefore, a weaker ionic or atomic bond of the target substance provides higher probability of being flicked by sputtering.
To form a transparent conductive film of an oxide such as ITO by sputtering, the following 2 methods are included; a reactive sputtering method for forming a film in a mixed gas of argon and oxygen, by using an alloy target of metals configuring the film (In—Sn alloy for an ITO film); and a reactive sputtering method for forming a film in a mixed gas of argon and oxygen by using the oxide sintered body made of oxides of metals configuring the film (In—Sn—O sintered body for an ITO film).
A method using an alloy target requires increased oxygen gas flow, because all oxygen in the resultant transparent conductive film is fed from oxygen gas in atmosphere; resulting in difficulty in maintaining small variation of oxygen gas amount in atmosphere gas. This method makes difficult to manufacture a transparent conductive film having consistent thickness and consistent characteristics, because film formation speed or characteristics (resistivity and transmittance) of the resultant film extremely largely depends on oxygen gas amount introduced into the atmosphere (see “Transparent conductive film technology”, Japan Society for the Promotion of Science, published by Ohmsha, Ltd., page 173, published in 1999).
On the other hand, a method using an oxide target is capable of reducing variation of oxygen gas amount in the atmosphere gas as compared with using an alloy target, because a part of oxygen supplied to the film is supplied from the target itself, and only deficient oxygen amount is supplied as oxygen gas; resulting in easier manufacturing of the transparent conductive film having consistent thickness and consistent characteristics as compared with using the alloy target. Therefore, a method for using the oxide sintered body as a target is widely adopted industrially.
In the same way as for a tablet for an ion plating method, use of a tablet made of the oxide sintered body is capable of manufacturing a transparent conductive film having consistent thickness and characteristics.
As described above, in consideration of productivity or manufacturing cost, a DC sputtering method is easier in high-speed film formation than a high frequency sputtering method; namely, a DC sputtering method provides about 2 to 3 times higher-speed film formation when compared by charging the same power to the same target. In addition, charging of high DC power is preferable to enhance productivity, because charging of higher DC power provides higher-speed film formation, even in a DC sputtering method. Therefore, such a sputtering target is industrially useful that is capable of stable film formation without generation of sputtering abnormality, even by charging high DC power.
By the way, an indium oxide-based material such as ITO or the like is widely used to obtain a transparent conductive film as described above, however, a non-indium-based material is required, because an indium metal is a rare metal on the earth, and it has toxicity and affects badly to environment or a human body. As the non-indium-based material, a zinc oxide-based material such as GZO or AZO, and a tin-oxide-based material such as FTO or ATO are known. A transparent conductive film made of a zinc oxide-based material is industrially manufactured by a sputtering method, however, it has a defect of poor chemical resistance (alkali resistance and acid resistance). In addition, a transparent conductive film made of a tin oxide-based material, although excellent in chemical resistance, has defect of difficulty in manufacturing by a sputtering method, because of difficulty in manufacturing a tin oxide-based sintered body target having high density and durability.
As a substance for solving such defect, a Zn—Sn—O-based transparent conductive film has been proposed. This Zn—Sn—O-based transparent conductive film is a material excellent in chemical resistance, which has overcome defect of a zinc oxide-based transparent conductive film. To obtain this Zn—Sn—O-based transparent conductive film, for example, a method for film formation by high frequency sputtering using mixed powder of sintered ZnO and SnO, as a target, has been proposed (see JP-A-8-171824). The resultant transparent conductive film by this method is a crystalline thin film having a Zn2SnO4 compound and a ZnSnO3 compound as main components; and is a thin film with improved chemical resistance, which is defect of a ZnO-based transparent conductive film. However, because the film is formed by high frequency sputtering using a powder-like target, such a target is not capable of obtaining an excellent transparent conductive film in a DC sputtering method.
In addition, such a film has been proposed that has structure laminated with a transparent film of mixed oxides of zinc and tin, and a reflecting film made of chromium nitride, on a substrate, one on top of the other (see JP-A-2-145458). However, this transparent film of mixed oxides of zinc and tin is formed by a reactive sputtering method using a zinc-tin-based alloy target, and has poor characteristics reproducibility of the resultant film. JP-A-2-145458 only describes composition (Zn/Sn ratio) of the alloy target used, and does not describe structure thereof. In general, a manufacturing method for a metal oxide thin film by reactive sputtering using a metal target is more likely to provide significant variation of film composition and film characteristics, and reduce yield. Charging of DC power with a charged power density of as high as equal to or higher than 2.0 W/cm2 particularly provides significant variation of film characteristics and deteriorates productivity.
As described above, in consideration of productivity of a transparent conductive film, manufacturing cost reduction, and quality stabilization of film-formed products, it is desirable to obtain a transparent conductive film using the oxide sintered body as a raw material, and a DC sputtering method charged with high DC power, or an ion plating method.
However, there is no practical use of the oxide sintered body for a sputtering target, or the oxide sintered body for an ion plating tablet, to provide high-speed film formation of a Zn—Sn—O-based thin film; namely, a conventional oxide sintered body generates a problem of arcing generation or the like by charging high DC power, therefore the selection here is limited only to film formation by charging low DC power, or by a high frequency sputtering method which provides low-speed film formation, and thus high productivity cannot be attained.
In addition, generation of arcing during film formation causes generation of particles and may also reduce product yield. In addition, continued arcing generation not only inhibits film formation itself but also inhibits manufacturing of a high quality film because of generation of damage in a film itself.
Reactive sputtering using a zinc-tin-based alloy target, described in JP-A-2-145458, is more likely to attain high-speed film formation of a metal oxide thin film made of zinc and tin, however, stable manufacturing is inhibited because of large variation of film composition or conductivity or transmittance. Sputtering film formation using a Zn—Sn—O-based oxide target is promising in reducing variation of film characteristics, however, there is only a film forming example using high frequency sputtering as described in JP-A-8-171824, and there was no sputtering targets which can stably be used in DC sputtering which is widely used industrially and advantageous in high-speed film formation. Under these circumstances, such the oxide sintered body for a target or an ion plating tablet has been required that does not generate arcing even under charging of high DC power, and is capable of high-speed film formation.